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Initial recommendation for misaligned stub pipe counter-boring that led to reduced wall thickness

Issue number: AO-2010-089-SI-01
Who it affects: Aircraft equipped with Rolls-Royce plc Trent 900 series engines
Issue owner: Rolls-Royce plc
Operation affected: Aviation: Air transport
Background: Investigation Report AO-2010-089
Date: 01 December 2010

Safety issue description

On 30 November 2010 the ATSB had, in close consultation with Rolls-Royce and the UK Air Accidents Investigation Branch, established that the occurrence was directly related to the fatigue cracking of an oil feed stub pipe within the No.2 engine’s HP/IP bearing support structure. The ATSB identified the following safety issue:

Safety issue
Misaligned stub pipe counter-boring is understood to be related to the manufacturing process. This condition could lead to an elevated risk of fatigue crack initiation and growth, oil leakage and potential catastrophic engine failure from a resulting oil fire.

  • During the manufacture of the HP/IP bearing support assembly fitted to the No. 2 engine (serial number 91045), movement of the hub during the machining processes resulted in a critically reduced wall thickness within the counter bore region of the oil feed stub pipe.
  • It was probable that a non-conformance in the location of the oil feed stub pipe interference bore was reported by the coordinate measuring machine during the manufacturing process, but that the non-conformance was either not detected or not declared by inspection personnel, resulting in the assembly being released into service with a reduced wall thickness in the oil feed stub pipe.

Recommendation

Action organisation: Rolls-Royce plc
Action number: AO-2010-089-SR-012
Date: 01 December 2010
Action status: Closed

The Australian Transport Safety Bureau recommends that Rolls-Royce plc address this safety issue and take actions necessary to ensure the safety of flight operations in transport aircraft equipped with Rolls-Royce plc Trent 900 series engines.

Output text

Examination of components removed from the failed engine at the Rolls-Royce plc facility in Derby, United Kingdom, have identified the presence of fatigue cracking within a stub pipe that feeds oil into the High Pressure (HP) / Intermediate Pressure (IP) bearing structure. While the analysis of the engine failure is ongoing, it has been identified that the leakage of oil into the HP/IP bearing structure buffer space (and a subsequent oil fire within that area) was central to the engine failure and IP turbine disc liberation event.

Further examination of the cracked area has identified the axial misalignment of an area of counter‑boring within the inner diameter of the stub pipe; the misalignment having produced a localised thinning of the pipe wall on one side. The area of fatigue cracking was associated with the area of pipe wall thinning.

Safety Issue
Misaligned stub pipe counter-boring is understood to be related to the manufacturing process. This condition could lead to an elevated risk of fatigue crack initiation and growth, oil leakage and potential catastrophic engine failure from a resulting oil fire.

ATSB response:

 On 1 December 2010, the ATSB issued the following safety recommendation to Rolls-Royce.


 

 

Proactive Action

Action organisation: Rolls-Royce plc
Action number: AO-2010-089-NSA-046
Date: 02 December 2010
Action status: Closed

In response to the developing understanding of this safety issue, on 2 December 2010 Rolls-Royce issued NMSB 72-G595 to operators of the Trent 900 engine, which required the specialised examination, measurement and reporting of the stub pipe counter bore geometry in these engines. No assessment or engine rejection criteria were included in the NMSB.

A 20 flight cycle compliance limitation was specified for the completion of the oil feed stub pipe examination.

ATSB response:

Despite the initial Rolls-Royce action to release NMSB72-G595, the ATSB was concerned that the bulletin did not place assessment and engine rejection criteria on the measurement of the stub pipe counter bore geometry. In addition, the ATSB did not consider the 20 cycle limitation as adequately addressing this safety issue. The ATSB consulted with CASA, who initiated the actions as detailed below.


 

Proactive Action

Action organisation: Civil Aviation Safety Authority
Action number: AO-2010-089-NSA-013
Date: 02 December 2010
Action status: Closed

On 2 December 2010, CASA issued a maintenance direction to Qantas under Regulation 38 of the Civil Aviation Regulations 1988. That direction required that Qantas:

(a) Comply with Rolls-Royce plc Service bulletin number 72-G595 subsequent and any amendment or revision of it, within two cycles from the issue of this direction

(b) In the event abnormal or eccentric counter-boring of the tubes described in the service bulletin is identified, this must be recorded as a major defect of the engine

(c) Upon completion of compliance with the service bulletin an entry shall be made in the aircraft’s maintenance records stating what actions were taken to comply with the service bulletin and this direction

(d) Upon completion of compliance with the service bulletin a written report shall be furnished to [CASA] stating how the service bulletin and this direction were complied with and the outcome of compliance with the service bulletin.

ATSB response:

The ATSB is satisfied that the action taken by CASA adequately addresses the immediate safety of flight concerns in respect of Qantas operation of A380 aircraft equipped with Trent 900 series engines. Therefore the ATSB makes no recommendation in relation to this issue.


 

Proactive Action

Action organisation: Rolls-Royce plc
Date: 02 December 2010
Action status:

On 2 December 2010 Rolls-Royce issued Revision 1 to NMSB 72-G595. This revision incorporated assessment and engine rejection criteria for the measurement of potential counter bore misalignments, and in particular, a tightening of the compliance from 20 to two flight cycles.

ATSB response:

The ATSB is satisfied that the action taken by Rolls-Royce adequately addresses the immediate safety of flight concerns in respect of Qantas operation of A380 aircraft equipped with Trent 900 series engines.


 

Proactive Action

Action organisation: Qantas
Action number: AO-2010-089-NSA-014
Date: 02 December 2010
Action status: Closed

On 2 December 2010, Qantas advised that:

...in response to Service Bulletin RB211-72-G595 (Revision 1), and in line with ATSB Safety Recommendation AO-2010-089-SR-012, Qantas will conduct a focused borescope measurement inspection of the HP/IP turbine bearing support structure oil feed tube for concentricity of the counter-bore and inspection of the related components on its RB211 Trent 900 series engines. The inspection results will be sent to Rolls Royce for evaluation. Rolls Royce will then provide Qantas with formal confirmation as to the serviceability of the engine.

These inspections will take place within the next 24 hrs on engines in place on A380 aircraft currently in service, and before further flight on engines on aircraft not yet returned to service.

ATSB response:

The ATSB is satisfied that the action taken by Qantas adequately addresses the immediate safety of flight concerns in respect of the operation of its A380 aircraft equipped with Trent 900 series engines. Therefore the ATSB makes no recommendation in relation to this issue.

   
Current issue status: Adequately addressed
 
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Last update 08 January 2014